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BIG PHYSICS, BIG QUESTIONS –

Atomic acrobats bounce on magnetic mirror

By Roland Pease

THE world’s first magnetic “trampoline” for atoms has been put through its paces in the US. The device, which allows supercooled atoms to bounce on cassette tape or a floppy disc, could be used to build precision clocks and flasks for storing cold atoms.

The system, developed by Ed Hinds, Ken Zetie and their colleagues at Yale University, relies on the fact that a neutral atom behaves like a miniature bar magnet, with a tiny magnetic dipole. Atomic dipoles of this sort get squeezed out of regions with high magnetic fields, if orientated in the right direction. So if a magnetic field has a sharp enough gradient in strength, it can reflect incident atoms like a mirror.

A working magnetic mirror has been slow to arrive. This is because at normal temperatures, atoms are too energetic to be stopped by magnetic fields. Also, no one had identified a way of creating the “sharp” magnetic field necessary.

Hinds and his colleagues hit on the right formula by using everyday cassette tapes. They recorded a high-frequency tone on a stretch of tape, causing its magnetic grains to align. This produced intense magnetic fields that decay exponentially just beyond the tape’s surface, so that the mirror “front” was just 1.5 micrometres deep.

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The researchers then drained energy from a cloud of rubidium atoms using a magneto-optic trap, a recently developed device that can trap and cool atoms to temperatures just above absolute zero. They chilled around 10 million atoms to a mere 30 microkelvin, suspending them about 2.5 centimetres above the magnetic mirror.

When the trap was switched off, the atoms fell onto the magnetic mirror. As they dropped, a weak magnetic field aligned their dipoles. The atoms bounced back up and down for around half a second (Physical Review Letters, vol 75, p 629), but then dispersed because of thermal expansion. “The atoms spread out unchecked and simply miss the mirror after a short time,” says Hinds.

Hinds and his colleagues found the perfect solution in the form of a magnetised computer floppy disc, distorted into a concave shape. When rubidium atoms were dropped onto this concave mirror, it refocused them on each reflection. Hinds says that the cloud of atoms then bounced on for several seconds before dispersing.

The experiment is more than just a party trick. In future, magnetic mirrors could be used to focus moving atoms into narrow beams. These could deposit tracks onto tiny electronic circuits, for instance. The idea could also be applied in atomic clocks, in which precisely timed electronic transitions in atoms can be “read” off, so long as the atoms are able to hover undisturbed. Atoms trapped in perpetual bouncing motion could provide the longest read-off periods possible.

Zetie says that the magnetic mirrors could also be used to create “thermos flasks” for storing cold atoms or rare isotopes. The inside of the flask would be covered in the magnetic material which could hold the atoms in space indefinitely, constantly bouncing off the walls (see Diagram).